The disclosure relates to roll forming of sheet glass, and more particularly to a method and apparatus for precision roll forming thin glass sheets with high precision thickness/dimensional uniformity, and more particularly to such a method and apparatus in which a stream of molten glass at a relatively high glass temperature greater than about 1000° C. or higher is formed by a first forming roll pair at a relatively high/hot surface temperature (500° C. or higher) and is then precisely sized by a precision dimensioned roll pair at a relatively low/cold surface temperature (400° C. or lower).
Historically, rolled sheet glass is typically formed using one pair of hot forming rolls. The first pair of forming rolls can be oriented such that the formed sheet glass exits the rolls either horizontally or vertically, or sometimes at angles between horizontal or vertical. In a conventional horizontal or angled glass forming system, a stream or sheet of hot molten glass is typically delivered to the lower forming roll (just ahead of the nip of the rolls) from, for example, an overflow weir. In a vertical glass forming system, a stream or sheet of hot molten glass is typically fed vertically down onto one of the forming rolls from a slot orifice or overflow weir. The exiting glass sheet can be fed directly into a heat treatment lehr or roller hearth to anneal the newly formed sheet.
Conventional rolling machines are normally single-purpose pieces of equipment that are relatively large and bulky machines built for a single glass forming process that produces a single type of glass sheet within relatively small compositional, viscosity, width and thickness ranges. Moreover, conventional glass roll forming machines that typically employ a single pair of hot forming rolls, produce a sheet glass that does not have a high precision of dimensional uniformity, e.g. does not have thickness uniformity at a high level of precision within +/−0.025 mm″) and unable to form thin glass below 2-3 mm in thickness. One factor that contributes to this lack of precise thickness control is the uneven thermal expansion of the hot forming rolls. When the hot forming rolls are heated to their operating surface temperature of 500° C. or higher, the hot forming rolls expand/swell. The uneven thermal expansion of the hot forming rolls makes it difficult to maintain a precisely dimensioned cylindrical glass forming surface on the hot forming rolls. As a result, the sheet of glass exiting the hot forming rolls does not have a high precision of thickness uniformity.
Compounding the thermal expansion problem with conventional hot forming rolls is the fact that heat from the high temperature of the molten stream of glass, which may be about 1000° C. or higher, raises the temperature of a central portion of the hot forming rolls to a higher temperature than the end portions of the hot forming rolls, causing the central portion of the hot forming rolls to expand more than the end portions. This uneven expansion of the hot forming rolls can be at least partially compensated for by forming the central portion of the hot forming rolls with a smaller diameter, such that the central portion of the hot forming rolls have a concave bow at room temperature. The concave bow can be designed to substantially counteract the greater thermal expansion that occurs in the central portion of the hot forming rolls at the elevated operating temperatures during glass fabrication compared to the thermal expansion of the end portions, such that when the hot forming rolls expand at operating temperature the rolls will be substantially cylindrical. Selective heating and cooling of the central and end portions of the hot forming rolls, as is well understood in the art, may additionally or alternatively be employed to minimize the temperature and thermal expansion variations in the hot forming rolls during hot roll forming of glass. Nevertheless, even with such steps, it has heretofore been extremely difficult if not impossible to form thin glass with a high precision thickness uniformity using forming rolls.
Fusion draw forming of glass produces wide, thin glass sheets with a high precision of thickness control and surface finish, such as Eagle™ glass from Corning Incorporated. However, fusion forming is typically limited to the formation of glass having viscosity above about 100,000 poise or more and produces sheets with relatively thick undesirable “edge beads” that need to be removed.
There is a need in the art for a precision glass forming process and apparatus that is capable of precision forming glass and glass ceramic compositions that cannot be fusion formed. There is also a need in the art for a process that is capable of precision forming thin glass sheets at a higher flow rate (process speed) than is possible with current fusion forming and roll forming processes. There is also a need in the art for a roll forming process and apparatus that is capable of precision forming a wide range of glass compositions having a wide range of viscosities into thin glass sheets having a relatively high precision thickness uniformity.